Light profile microscopy is an imaging method for obtaining direct images of cross sections of thin films using a light profile microscope (LPM) (see for example U.S. Pat. No. 6,614,532). The cross sectional images thus obtained may be used to identify the number, dimensions, composition and/or material morphology of individual layers making up the structure of a thin film test material for example. Light profile microscopy is to date a unique method for direct micrometer scale imaging of depth structures in thin layers, which may be conducted rapidly and with minimal modification and/or preparation of the test material. Light profile microscopy furthermore allows an image resolution that is close to or equal to the Rayleigh diffraction limit of an optical microscope as defined herein below (see relations 5 and 6 below).
In a standard set up for light profile microscopy, a material under test, referred to hereinafter as the test material, is usually, although not necessarily, a planar structure with major lateral dimensions and a depth dimension, along axes designated as ‘y’, ‘z’ and ‘x’ respectively. A beam of excitation radiation, referred to hereinafter as the source beam, is directed through the test material along the depth dimension ‘x’. This beam is displaced along the ‘z’ axis behind a cross sectional surface of the test material, which may be an edge prepared by cleavage to expose the cross sectional structure of the thin film. This surface is referred to as the image transfer (IT) surface because radiation transferred through it is used to form an image of the excitation beam propagating behind it, inside the test material.
The IT surface is usually polished to prevent any optical defects present thereon from affecting the image. Image radiation is emitted from the source beam in the test material by light scattering, which may be elastic Rayleigh scatter, or inelastic Raman scatter, by luminescence (fluorescence and/or phosphorescence), or by other emission mechanisms, such as blackbody emission, for example, or by chemi-luminescence as excited thermally by the source beam. The image radiation is typically, although not exclusively, incoherent with the radiation in the source beam.
An optical imaging system (OIS) is used to form the light profile microscopy (LPM) image from the image radiation. The OIS typically comprises a combination of lenses and/or mirrors forming an image at an image plane. The OIS is aligned at ninety degrees to the depth axis ‘x’ along a direction normal to the IT surface, which typically corresponds to the ‘z’ direction, hereinafter referred to as the optic axis.
The LPM image is recorded with a spatial resolution that is close to the diffraction limit of the OIS, as set forth hereinbelow. The OIS is assumed to have its image resolution close to the diffraction limit. It is to be noted that the imaging properties of the OIS differ from those of macroscopic scale imaging systems. The latter systems have image resolution that is far from the diffraction limit and is limited severely either by optical aberrations or by the dimensions of the image pixels of a camera or image recording instrument.
The LPM image has a number of features characteristic of images recorded using an OIS in such an LPM layout. First, the limited object depth of focus of the OIS allows forming an image of the source beam, with the source beam aligned at a sufficient distance behind the IT surface, inside the test material, so that any scratches and defects in the IT surface are held significantly out of focus in the LPM image. It is to be noted that in the event that this condition is not strictly met, the LPM measurement is not invalid. Second, the orthogonal LPM geometry maintained between the depth axis ‘x’ and the optic axis ‘z’ yields a very high contrast in the LPM image for interfaces and boundaries in the cross section of the thin film test material. This image contrast is much greater than that available from images obtained using other microscopy methods known in the art. This orthogonal LPM geometry allows making direct imaging of depth structures in a material. Third, the image features of structures that scatter light in the direction of the OIS are emphasized in LPM images, in contrast to features that do not so scatter, which allows LPM images to record structures that may appear invisible in other prior art microscopy methods. This high scatter contrast may also have the effect of rendering insignificant scratches and defects in the IT surface and their contribution to LPM images that are recorded when the source beam is close to the IT surface.
A number of applications of light profile microscopy as a method of industrial thin film imaging are described in U.S. Pat. No. 6,614,532 by the present inventor for example, and in recent publications in the literature (see for example J. F. Power and S. W. Fu, Longitudinal Light Profile Microscopy (LLPM): A New Method for Seeing Below the Surfaces of Thin Film Materials Applied Spectroscopy 53(12), 1507-1519 (1999); S. W. Fu and J. F. Power, Broadband Light Profile Microscopy (BB-LPM): A Rapid and Direct Method for Thin Film Depth Imaging, Applied Spectroscopy 58, 96-104 (2004); J. F. Power and S. W. Fu, Dual Beam Light Profile Microscopy (LPM): A New Technique for Optical Absorption Depth Profilometry, Appl. Spectros. 58(2), 166-178, (2004)).
However, there is still a need in the art for a method and apparatus of light profile microscopy providing an optimized image resolution under broad field conditions.